It has been hypothesised, based on Vostok ice core CO2 data (Petit et al., 1999), that atmospheric CO2 would have dropped naturally by 20 ppm during the past 8 kyr (in contrast with the observed 20 ppm increase) if prehistoric agriculture had not caused a release of terrestrial carbon and CH4 during the Holocene (Ruddiman, 2003; Ruddiman et al., 2005). This hypothesis also suggests that incipient late-Holocene high-latitude glaciation was prevented by these pre-industrial greenhouse gas emissions. However, this hypothesis conflicts with several, independent lines of evidence, including the lack of orbital similarity of the three previous interglacials with the Holocene and the recent finding that CO2 concentrations were high during the entire Stage 11 (Siegenthaler et al., 2005a; Figure 6.3), a long (~28 kyr) interglacial (see Section 6.4.1.5). This hypothesis also requires much larger changes in the Holocene atmospheric stable carbon isotope ratio (13C/12C) than found in ice cores (Eyer, 2004), as well as a carbon release by anthropogenic land use that is larger than estimated by comparing carbon storage for natural vegetation and present day land cover (Joos et al., 2004).